FIG. 1 shows schematically the configuration of a typical wireless cellular telecommunications network. The network comprises a number of base-stations (BSs) 4, 5, 6 etc. Each base-station has a radio transceiver capable of transmitting radio signals to and receiving radio signals from the area of an associated cell 1, 2, 3 etc. By means of these signals the base-stations can communicate with a terminal 9 which may be a mobile station (MS) in the associated cell. That terminal itself includes a radio transceiver. Each base station is connected via a base station controller (BSC) 7 to a mobile switching centre (MSC) 8, which is linked in turn to the public telephone network (PSTN) 10. By means of this system a user of the mobile station 9 can establish a telephone call to the public network 10 via the base station in whose cell the mobile station is located. The location of the terminal 9 could be fixed (for example if it is providing radio communications for a fixed building) or the terminal could be moveable (for example if it is a hand portable transceiver or mobile phone).
If the mobile 9 moves from one cell to another there is a need for it to switch from communicating with one base station to communicating with another. This process is known as handover. As an example, at location 11 the mobile 9 communicates with base station 4. if the mobile 9 moves from location 11 to location 12 along route 13 then at some point it must hand over from communicating with base station 4 to communicating with at least one of base stations 5 and 6. When the mobile station 9 is communicating via base station 4 with another terminal unit 14 the communications data passes across the radio link between the mobile and the base station 4, fixed wire link 15 between the base station 4 and the corresponding BSC 7 and then onward to terminal 14.
As the mobile moves from one cell to another the decision on when to hand over, and to which base station to hand over, is conventionally based on estimates of the quality of the radio links between the mobile and each of the candidate base stations, including the one with which it is currently communicating. A number of measures of quality are available; examples of available measures include frame error rate, bit error rate, signal to noise or signal to interference rate (SNR or SIR), received power of one unit at the other and path loss. These measurements may be made for communications in either or both directions across the radio link. The handover process may be initiated or requested by the mobile station but it is normal for the ultimate decision on when to hand over to be taken on the network side of the air interface. This is because the network has knowledge of which of the candidate base stations has available capacity for communicating with the mobile and can take this, in addition to link quality, into account in making the handover decision. Therefore, mobile stations are normally required to report to the network the results of their quality measurements of the candidate base stations. The result in most systems is that—subject to relatively minor sources of error such as rapid variations in quality that are not accommodated due to delays in the handover process and subject to network constraints such as capacity—the link quality between the mobile and the unit with which it is communicating is kept substantially optimised.
Moves are being made to integrate networks of the type shown in FIG. 1 with other signalling networks such as intranets or the internet. This has the advantage that it may avoid the need to install dedicated synchronous links for the mobile network. Instead, using a protocol such as H.323 voice traffic from one terminal to another can be sent over existing packet switched links. These links may, for instance be between a base station and its BSC or between a BSC and the remainder of the network. This has significant advantages in reduced cost of setting the network up and maintaining it. Also, if the network is, for example, a company's intranet then it can be used to route calls within the company without the need for the company to pay a network operator for the use of an external network. However, problems may be encountered with handover in such a system. When a mobile station is handed over from one base station to another its user may experience a significant decrease in performance because, although the radio link to the new base station is better than that to the old base station, there are other problems in the network that cause the network-side links to the new base station to be worse. These problems could, for example, be high usage of the network in the region of the new base station or its BSC that results in packet loss, or routing of packets to or from the new base station by a long route that introduces delays or jitter.
There is thus a need to improve handover performance in telecommunications networks.